Parkinson's disease (PD) is a common progressive neurodegenerative disorder. Current therapeutic approaches initially alleviate symptoms but eventually cause deleterious side effects and fail to halt disease progression. A significant genetic component to disease was recently identified as mutations in the leucirierich repeat kinase 2 gene (LRRK2). LRRK2 mutations cause a highly-penetrant dominant disease phenotype indistinguishable from typical PD. Preliminary data suggest that LRRK2 mutations perturb the normal enzymatic activity of LRRK2 by increasing kinase activity. Such increases in kinase activity are associated with neurotoxicity. The mentored phase of this proposal dissects LRRK2 toxicity using a combination of approaches including inducible gene expression, RNA interference, and viral-delivery of mutant LRRK to primary neurons. The goal is to define LRRK2-mediated kinase dependent cell death cascades. Complementary to understanding the role of LRRK2 in health and disease will be the identification of LRRK2 kinase substrates in relevant cells. As the first of two independent phase projects, LRRK2 kinase substrates will be identified using a combination of novel technologies to assess the complete set of LRRK2 interacting proteins in an unbiased manner. The functional impact of LRRK2 mediated kinase activity on protein substrates will be evaluated, particularly in existing and emerging models of PD. Finally, the second independent phase aim utilizes the tools and techniques developed in this proposal to perform high-throughput screening to identify small-molecule LRRK2 kinase inhibitors. Small molecule inhibitors will be used to definitively assess the role of LRRK2 kinase activity in causing neurotoxicity. LRRK2 may represent a far upstream element in the pathogenesis of PD. Through the understanding of LRRK2, other genes involved in PD may fall into a common biochemical pathway where disease intervention is possible. Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene are a common cause of Parkinson's disease. This proposal explores the role of LRRK2 in neurodegeneration with a focus on identifying pathways and drugs to intervene in the disease process.